Abstract

Using a combination of experimental techniques such as optical absorption, Raman scattering, continuous wave and pulse Electron Spin Resonance (ESR), we characterize a set of γ-irradiated Yb3+ doped silica glass preforms with different contents of phosphorous and aluminum. We demonstrate that when P is introduced in excess compared to Al, nearly no radiodarkening is induced by γ-rays. On the other hand, when Al>P, a large absorption band is induced by radiation. Thermal annealing experiments reveal the correlation between the decrease of the optical absorption band and the decrease of the Al-Oxygen Hole Center (AlOHC) ESR signal, demonstrating the main role of AlOHC defects in the fiber darkening. HYSCORE (HYperfine Sublevel CORElation) pulse-ESR experiments show a high Al-P nuclear spin coupling when P>Al and no coupling when Al>P. This result suggests that both AlOHC and POHC creation is inhibited by Al-O-P linkages. Confronting our data with previous works, we show that the well-known photodarkening process, meaning losses induced by the IR pump, can also be explained in this framework.

(a) Optical absorption spectra of the seven irradiated preforms. Samples A and B (P>Al) exhibit low absorption band in the visible range. The sharp peak around 980 nm is the absorption band of Yb3+. The spectra have been cut between 850 nm and 930 nm due to discontinuities induced by the change of the lamp during the experiment; (b) cw-ESR spectra of the seven irradiated samples. The inset displays an enlargement of the weak signal of AlOHC defects around 348 mT. All the spectra have been recorded at ambient temperature.

(a) cw-ESR spectra of the preform G at different power between 0.1mW and 50 mW around the AlOHC defect resonance; (b) Intensity of the AlOHC signal in function of the power square root. A linear behaviour is observed up to P = 2mW, the working power.

(a) Evolution of the intensity of the visible absorption band; (b) the intensity of AlOHC ESR signal during the thermal annealing experiment. Both characterizations display a stable behaviour up to 200 °C, and a decrease of the signal between 200 °C and 400 °C. Above 450 °C, the visible absorption band and the AlOHC ESR signal have completely disappeared.

2D-HYSCORE pulse ESR spectra of the pristine samples A, B, D, E, F, and G recorded at liquid He temperature and at a magnetic field of 350 mT. A and B samples reveal off-diagonal peaks showing the proximity of Al and P nuclei around Yb3+ when phosphorous is introduced in excess compared to aluminium.